Many members of Enterobacteriaceae and Pseudomonas aeruginosa (subject of this grant application) have the ability to sense damage inflicted to their cell wall by -lactam antibiotics. A primary mechanism for this sensing involves the events of cell-wall recycling, and results in the induction of resistance mechanisms. These events have led to the obsolescence of many of the -lactam antibiotics against these Gram-negative bacteria. These multiple complex steps are poorly elucidated, and are the subject of this grant proposal. Three Specific Aims are proposed. In Specific Aim 1 I propose to elucidate the reactions of all lytic transglycosylases, enzymes that initiate the cell-wall recycling events. This undertaking utilizes methodologies developed in my lab for highly sensitive high-resolution identification of reaction products of these enzymes by proteomics approaches. Furthermore, a link between the function of penicillin-binding protein 4 (PBP4) of Pseudomonas aeruginosa and the sensing of the presence of the -lactam antibiotic has been made. I disclose my views on how this process could take place and I propose the means to the identification of the signal molecule, which triggers the antibiotic resistance processes. Specific Aim 2 proposes to study and identify a key enzyme in the resistance induction process, the AmpD protease. Three homologous enzymes have been annotated in P. aeruginosa for this activity. We have outlined studies that will delineate which of the three, or a subset thereof, are bona fide enzymes involved in cell-wall recycling and antibiotic resistance induction. Specific Aim 3 will delineate the transcriptional events that lead to -lactam antibiotic resistance in P. aeruginosa. This includes identification of the key transcriptional activator molecule and the kinetics of the processes. I anticipate that the successful completion of this proposed science will not only lead to the elucidation of these events, but also will identify opportunities for their interruption as means to circumvent the elaborate mechanisms of resistance.